The present invention relates to an array type ultrasound type probe and a manufacturing method and a driving method of the array type ultrasound probe used for medical diagnosis.
An ultrasound diagnosis apparatus is a medical imaging apparatus which obtains a tomography image of a soft tissue of a biological object through ultrasound pulse reflection method without damages. The ultrasound diagnosis apparatus is compact, inexpensive and safety without causing irradiation by X-ray, compared to other medical imaging apparatuses. It is widely used in departments of cardiovascular disease (arteria coronaria of the heart), digestive organs (gastrointestinal), internal medicine (liver, spleen and pancreas), urology (kidney and bladder) and obstetrics and gynecology. Such ultrasound probe used for the ultrasound medical diagnosis apparatus generally utilizes a piezoelectric effect of a piezoceramic so as to perform transmission and reception of ultrasound with a high sensitivity and a high resolution. In this case, as a vibration mode of the piezoelectric element for transmission and reception, a single type prove and an array type probe in which a plurality of probes are disposed two-dimensionally are popularly used. Since array type can obtain a fin image, it is popularly used for the medical image for diagnostic examination On the other hand, since harmonic imaging diagnosis using a harmonic signal can obtain a clear image which conventional B mode diagnosis cannot obtain, it is becoming a standard diagnosis modality. A harmonic imaging technology is a technology to improve a resolution of a ultrasound image and to form a fine image, where the ultrasound probe transmits a fundamental wave (f1) having a certain frequency to an organic object and receives a harmonic wave reflected by the organic object having frequency components, which are integral multiplication (for example, 2 times, 3 times, 4 times and 5 times) of frequency of the aforesaid fundamental wave, and various image processing are carried out after the ultrasound diagnosis apparatus converts the reflected wave into an electric signal. This harmonic imaging technology is also utilized to extract a diagnosis region as an image where a contrast is emphasized by dosing an ultrasound imaging agent including micro bubbles through an artery. The reason is that the micro bubbles dosed into the organic object has a characteristic to reflect the harmonic wave strongly.
It is necessary to improve the image quality of the ultrasound image of the diagnosis region using the harmonic imaging technology also for the ultrasound diagnosis apparatus using an ultrasound probe in which the vibration elements are disposed two-dimensionally. For this purpose, the ultrasound probe in which the vibration elements are arranged two-dimensionally is required to have a frequency characteristic of broad band including frequency components of the fundamental wave and the harmonic wave.
The harmonic imaging has various advantages where a contrast resolution is improved because of a superior S/N ratio due to a small side lobe level, a horizontal resolution is improved since a beam width is thin due to a high frequency, multi reflection does not occur because a fluctuation of a acoustic pressure is small due to a small acoustic pressure at close range, and a diagnosis range can be deeper compared to an ultrasound where a decay beyond a focus is a similar level as the fundamental wave and the fundamental wave has a frequency of the harmonic wave.
As a substantial structure of the array type ultrasound probe for the harmonic imaging, an piezoelectric vibration element where each vibration element configuring an array is a broad band integrated type. There is generally used a method where the fundamental wave is transmitted in a frequency area of a low frequency side of the broad frequency band characteristic and the harmonic wave is received in a high frequency side of the frequency range. Under this circumstances, in conventional ultrasound probes, a technology to improve a sensitivity disclosed in Patent document 1 is know. This technology is to improve the sensitivity. In this technology, the vibration element where microscopic pillar-shaped piezoelectric elements bonded by an organic compound such as, for example, an epoxy resin, is used as an ultrasound transmission-reception element, and each pillar-shaped ceramic is vibrated in longitudinally.
A narrow frequency range ultrasound is used so that a spectrum of the ultrasound for transmitting the fundamental wave and a spectrum of the ultrasound for the receiving harmonic wave do not overlap each other. However, since the narrow frequency range ultrasound is generally an ultrasound pulse signal having a long tail, it negatively affects a resolution in a direction of ultrasound emitting.
As other practical structure of array type ultrasound probe for harmonics imaging, for example, a transmission-reception separation type probe where the transmission piezoelectric vibration element and the reception piezoelectric vibration element are separately disposed is suggested, in Patent document 2 and 3.
For example in Patent document 3, to transmit the fundamental wave and to receive the ultrasound including the harmonic wave, it is suggested that a first piezoelectric layer configured by a plurality of arrayed piezoelectric element having a first acoustic impedance to carry out transmission and reception of the ultrasound having a center frequency of f1, and a second piezoelectric layer configured by a plurality of arrayed second piezoelectric elements having a second acoustic impedance and laid on the first piezoelectric layer to carry out reception of the ultrasound having a center frequency of f2=2×f1 are disposed. However a sufficient sensitivity is not yet obtained.
Further, to improve the sensitivity it is being practiced that a piezoelectric ceramic element is laminated to lower an apparent impedance so that an electrical matching condition with a driving circuit is improved, and an electrical field intensity is increased to create a large distortion so that the transmission sensitivity is improved. However, in an laminated structure, though the remittance sensitivity is increased in accordance with number of the lamination layers, the reception sensitivity is inversely proportional to the number of the lamination layers, thus it is not preferable for the harmonic imaging.
[Patent Document 1] Unexamined Japanese Patent Application Publication No. 63-252140
[Patent Document 1] Unexamined Japanese Patent Application Publication No. 8-187245
[Patent Document 1] Unexamined Japanese Patent Application Publication No. 11-276478
To manufacture the composite piezoelectric vibration element, a piezoelectric ceramic is cut by a cutting machine such as dicer to form a pillar-shaped structure, thereafter a cutting groove is filled with an organic material such as an epoxy resin. Also, in case of the array type probe, separation of each channel is done by the cutting machine such as the dicer.
However, since an transmission-receiving frequency depends on a thickness of the piezoelectric ceramic, the pillar-shaped structure or a channel pitch becomes smaller as the frequency becomes higher. Therefore, there were problems that a mechanical strength of the ceramic is deteriorated in accordance with increase of machine work by cutting machine such as the dicer and a deterioration of properties due to heat and distortion at machining or breakage cannot be ignored, thus a deterioration of an production yield or deterioration of performance of the vibration element or the probe, become easy to be caused. Also, to manufacture the transmission-reception piezoelectric element capable of a desired frequency, it is indispensable to grind and polish both surfaces and is a factor to rise a production cost. As above, there were problems to overcome in the production of the probe using ceramic materials.